A multiple-input, multiple-output ([in English:] multiple input, multiple output, MIMO) communication system uses many transmission antennas and many reception antennas for implementation of data transmission. A MIMO channel is formed by the transmission and reception antennas and can be broken down into independent channels. Each of these independent channels is also referred to as a spatial sub-channel or inherent mode of the MIMO channel. Such systems are particularly known from mobile radio technology and are described in EP 1 117 197A2, for example.
MIMO (Multiple Input Multiple Output) refers, in communication technology, to the use of multiple transmission and reception antennas for wireless communication. This is the basis for special coding methods that make use of not only the time dimension, but also the space dimension for data transmission (Space-Time Coding). The principle that is used in MIMO comes from military radar technology, which has already been in use for many years. There, not just one but multiple antennas having the same construction are used. The antennas have a distance of at least half a wavelength (lambda/2) of the carrier frequency from one another. A third dimension is added to the frequency-time matrix that has been usual until now. In this connection, the data signal is transmitted by way of multiple antennas. At the same time, multiple reception antennas are also used. The signal-processing reception unit receives spatial information by means of multiple radio signals. This is because the same radio signal arrives at the receiver from two different directions, at two antennas. Each incoming radio signal generally has its own “spatial fingerprint,” which is also called a “spatial signature.” The receiver assembles the signals again, in suitable manner. This significantly improves the performance of the entire radio system.
As a result, quality, bit error frequency, and data rate of a wireless connection can be clearly increased. MIMO systems can transmit significantly more bit/s per Hz bandwidth used, and thus have a greater spectral efficiency than conventional SISO or SIMO systems.
The simplest MIMO hardware consists of two transmission antennas and one reception antenna. In order to optimally utilize the performance capacity, antennas are always used in pairs. The MIMO signal processing algorithms are thereby simplified and lead to an optimal signal-to-noise ratio.
The bandwidth can be increased, in linear manner, with the number of transmission antennas. Separation of the individual signals is a simple linear matrix calculation, which is calculated by high-performance processors. If one proceeds from this calculation, then theoretically, the transmission capacity could be increased infinitely. Eight transmission and reception antennas, in each instance, are considered to be the maximum. The advantages of multiple-antenna systems are the following:                greater reception power (group benefit)        interference suppression (interference suppression benefit)        better connection quality (diversity benefit)        higher transmission rates (multiplex benefit)        
High-frequency transmission techniques are used for data transmission, in particular; their wavelengths normally lie at approximately 18 GHz, 30 GHz, and 42 GHz.
However, in order to be able to transmit high-rate data, in general optical transmission is preferred. In order to allow optical data transmission and tolerances in the case of moving transmitters and receivers (platforms), adaptive optics are preferred. However, this embodiment is very cost-intensive and, in particular, sensitive to ambient influences.
A significant advantage of high-frequency transmission lies in that data transmission is possible even under poor weather conditions, particularly at an adaptive data rate. As a result, moving platforms on which transmitters or receivers are disposed can be tracked accordingly. However, the sensitivity to interference is very great, so that—as is known from terrestrial or space communication—it is necessary to make use of a laser communication system.
These optical systems bring the advantage that they are suitable for high-rate data transmission. At a wavelength of 0.5 to 2.2 μm (micrometers), the data transmission is very great; however, it is very dependent on weather conditions, in particular, and other optical influences.